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A/C advice needed!

1381 Views 6 Replies 2 Participants Last post by  mojobenz
My air suddenly stopped blowing cold on my 93 500 SEL. Checked fuses, they're good. Then had my local mechanic recharge and add Super Seal Premium to system in case there were any leaks. He said compressor still wouldn't kick on, though it looks fairly new. He checked electrical (as best he could I suppose), and couldn't find any problems. Anybody encountered this before?
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More info on the problem --- compressor is not locked up or anything. It spins free, but it's not getting electricity. Fuses are good. Are there any inline fuses or relays that need to be checked?
I think you need more than a shade tree mechanic, I know the 92 and I think the later models had more sensors than a cardiac ward, everything from a speed sensor that would shut it down if the belt was slipping, to low on r134, and of course the common leaking evaporator. (dash out $4K out of your wallet. One service bulletin shows how to jumper the low pressure switch to force the compressor on (but only for a quick)test to see if the system has any charge. All you are seeing is the pulley spinning if you cant get the clutch on the pulley to engsge the compressor will not "compress" anything
Jim's page shows how to pull codes from yout dash temp controls. The blower is controlled by a solid state regulator mounted on the fan motor you did not say it was not blowing, then you have different problems
Thanks, Peter. I was hoping you'd see my thread, since we have nearly the same car and you are quite knowledgeable. I got the idea for the Super Seal from Jim's page. Since I posted this, I did a face to face consult with the MB indy who rebuilt my transmission. He said these "overengineered" cars (though not as overengineered as the newer ones) have that control module for the compressor which senses engine speed. It's not the blower regulator, fan's working fine. If it's keeping the R134a, he said it's not the evaporator. He will diagnose the problem next week, because his shop stays full of MB, BMW, Audi and Porsche products.
Peter, thank you for your tips on the forum. Since you are knowledgeable about these cars, what is your take on these results of my test?
On the sensor values, refrigerant pressure shows OO, though system was charged without apparent leakage. Is it the dreaded evaporator, or is the sensor just bad? I don't want to get ripped off by the mechanic.
My only fault codes were 75, 83 and 87, which deals with heat supply unit circulation pump, left duovalve and right duovalve. My heater and blower appear to be working fine. Old codes which weren't cleared, maybe?
He is correct, my 92 had a bad belt tensioner, and whenever I accelerated hard or went thru a big puddle tha AC would shut down due to the sensor. This after having the evap replaced, you had to actually turn off the car to get it to reset, I complained and it was "oh yea" we know about that problem. As I stated on another post if you have 00 you have no residual pressure and the low pressure switch will not allow it to come on, there is a proceedure to junmp the switch to see if there is any rise in the sight glass. I have a relative in the foreign auto repair business, most of my work goes to MB
I have gained a lot of knowlege on this beast in the last 13 years, also spent a lot of money, never had a car this long in my life.
My 85 300TDT was the best, my 89 SEC came second, my 89 SL gave me some grief with AC and MB warranty, MY 98 ML was junk. the 01 E wagon is great. All my American cars gave less problems and my wife is in love with the SEL...
Just to wrap this up, for the record and to help other owners, my problem turned out to be the refrigerant pressure switch, about a $50 part. Replacing that and recharging the system with R134a solved the problem. Refrigerant pressure reading, 09, so no evaporator problem.
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